This invention relates to a novel form of phosphate phosphor activated by gadolinium.
Ultraviolet rays included in the sunlight perform very important effects for animals. One of these effects is a sterilizing effect and the other is the so-called erythematic effect, that is, an effect of giving rise to sunburns on the skin. Erythema is generated by ultraviolet rays having a wavelength of 280 to 330 nm, and mainly at a wavelength approaching 310 nm. Ultraviolet rays falling within the above-mentioned wavelength range have the function of producing vitamin D from ergosterin due to the erythematic effect, that is, displays an antirachitic action. Further, such forms of ultraviolet rays promote the metabolism of an inorganic element such as calcium or phosphorus in a living organism. Ultraviolet rays having a wavelength of 280 to 330 nm carry out the above-mentioned functions are important for the health of human beings, and consequently are generally referred to as health rays.
In recent years, however, living environments devoid of sunlight are gradually increasing in large cities due not only to only construction of tall buildings, underground railroads, and underground streets but also pollution of the atmosphere by various contaminants. Therefore, the so-called health lamp using such a phosphor as emits the aforesaid health rays has come to be put to practical application for the object of compensating for the decrease in the period of time during which city dwellers receive the sunlight. Well-known among health ray-emitting phosphors used with such a health lamp is thallium-activated phosphate, particularly calcium zinc phosphate (CaZn).sub.3 (PO.sub.4).sub.2 /Tl. However, this phosphor has the drawback that it has a short life performance, that is, it tends to be quickly deteriorated soon.